The invention relates to lamps and, in particular, to LED tubes which have one or more LEDs as light sources and which can replace a fluorescent tube.
Fluorescent lamps are widely used in different objects, such as in homes, offices and industry. Fluorescent lamps are more durable, economical and efficient than incandescent lamps, in which most of the electric power turns into heat instead of light. In a traditional fluorescent lamp, the body is a straight tube with a length of 15 to 60 times the diameter of the tube. The tube may also be bent, in which case it may be of almost any shape. Fluorescent tubes are low-pressure mercury discharge lamps in which the inner surface of the tube is coated with fluorescent material. The structure of a fluorescent tube is very simple and is illustrated in FIG. 1A. The lamp consists of an air-tight glass tube 4 containing a small amount of mercury, an inert gas, a fluorescent coating (luminophor), such as phosphor, and electrodes 2 and 3. At each end of the fluorescent tube, there is a lid 5 or 6 with two symmetrically positioned contact pins 7 and 8 or 9 and 10, to which the electrode 2 or 3 is connected. Power supply to the fluorescent tube is provided via these contact pins 7 and 8; 9 and 10. When the lamp is in operation, the temperature of the electrodes 2 and 3 must be sufficiently high, so that electrons release from them. A fluorescent lamp does not go on at a normal operating voltage without preheating. It is typical of fluorescent tubes (EN 60081) that their cathodes are heated with separate preheat circuits or arrangements. On the other hand, after the lamp has gone on, the discharging current through the tube must be restricted, so that the tube will not be damaged. Therefore, all fluorescent tubes require some kind of ballast. Conventionally, the ballast has been a ballast-starter combination, which is illustrated in FIG. 1B. When mains voltage (e.g. 230 VAC) is switched on to the lighting fixture, the resistance through the tube is very high, and the electric current goes through a ballast L, the electrode 3, a closed starter 11, and the electrode 2. When going through the electrodes 2 and 3, the electric current heats the electrodes, causing them to emit electrons which ionize the gas inside the tube. The ionized gas forms a current path through the tube. The current going through the ballast L generates a magnetic field in the ballast. When, after a moment, the starter 11 opens, the magnetic field of the ballast L generates a high voltage between the electrodes 2 and 3, which switches the lamp on.
Nowadays, electronic ballasts are also used. The electronic ballast also responsible for lighting up the lamp, so there is no need for a separate starter. A preheating arrangement is provided by either separate preheating windings or a starter capacitor. This is illustrated in FIG. 1C. An electronic ballast 12 connected to the mains voltage (e.g. 230 VAC) provides a continuous electric current through each of the electrodes 2 and 3. These electric currents are configured in such a way that a voltage difference is generated between the electrodes 2 and 3. When mains voltage is connected to the ballast 12, the electric current that goes through the electrodes heats them quickly, and the emitted electrons ionize the gas in the tube. The gas having ionized, the voltage difference between the electrodes starts a gas discharge.
The intention is to replace fluorescent tubes with LED tubes having the same length and values. In these, the physical dimensions are the same as in straight fluorescent tubes (e.g. T8 with a diameter of 26 mm and a length of 60 or 120 cm), whereby the fluorescent tube could be directly replaced with a LED tube in an existing fluorescent lamp. Examples of LED tubes directly connectable to the mains with a ballast are disclosed in publications EP1852648 and U.S. Pat. No. 7,441,922. Examples of LED tubes equipped with an electronic ballast are disclosed in publications FI64487 and US2007/0183156. The electronic ballast usually supplies a high-frequency (20 kHz . . . 100 kHz) voltage to the fluorescent tube pins, and the control electronics of the LEDs rectify the voltage and limit the current to the LEDs appropriately. Other examples of LED tubular lighting fixtures are disclosed in publications US2010/0002439 and WO2009/131340. The aim is to achieve a long lifetime for the light source as well as improved luminous efficiency (amount of light/electric energy).
In practice, the intention is to replace a fluorescent tube with a LED tube without changing the lighting fixture structures. Some of the LED tubes work directly with a fluorescent tube ballast, in which case only the starter should be removed from service. Then, the LED tube can be replaced easily and without assistance from a professional.
This causes a few problems, the most significant of which is the risk of an electric shock during the mounting step of the LED tube. FIG. 2 shows a simplified conceptual drawing of a fluorescent tube lighting fixture 20 comprising a body 24 with required electric structures therein, such as the ballast/ballast 12 and the starter 11 that is usually required only in connection with a ballast. At the ends of the lighting fixture, there are tube supports 21 and 22 with contact caps 23 into which contact pins of ends 26 and 27 of a tube 25 are inserted to achieve mechanical and electric connection. According to the safety regulations in the field of electricity, lighting fixtures are to be constructed in such a way that, when a fluorescent tube is replaced, it is not possible for a user to touch any parts at the mains voltage even if the lighting fixture were carrying voltage. This requirement is met even if the fluorescent tube were replaced in such a way that only one end 27 of the tube 25 were in contact with the contacts 23 of the tube support 22 and the person replacing the tube can touch the other end 26 of the tube. This requirement is met because no current goes through the gas-filled fluorescent tube before the gas in the tube is ionized with a starting pulse. In other words, the gas in the fluorescent tube serves as an insulator in itself. The electric structure of the lighting fixture, in turn, is such that generation of a starting pulse requires that both ends of the tube be connected to the contacts of the tube support. This way the fluorescent tube prevents the risk of an electric shock during replacement.
With LED tubes, this electric safety requirement is not met. Inside LED tubes, there is a printed board or a corresponding structure, on which LEDs and electronic current supply components they require are mounted. The purpose of these components is to convert the alternating voltage of the mains into direct voltage and to regulate the direct current required by the LEDs. In practice, current flows through these components once voltage is applied to them, in other words, the LED tube is in a conductive state without a separate starting pulse. Therefore, in a practical situation, when the LED tube 25 is being mounted on a fluorescent tube lighting fixture 20, the contact pins 27 at one end of the LED tube 25 may hit the contacts 23 of the tube support 22, and the other end 26 of the tube may remain outside the lighting fixture, so that the person mounting or replacing the tube may touch it, whereby s/he is prone to the risk of an electric shook.
Another factor deteriorating electric safety is the cooling of a LED tube. Since the service life of LEDs is highly dependent on their operating temperature, various solutions have been sought for cooling such a LED tube. Some solutions suggest perforating the LED tube (e.g. U.S. Pat. No. 7,611,260), whereby air flows through the holes transferring heat from the LEDs out of the tube. In such a solution, owing to the plastic body of the LED tube, high insulation level of the live parts is still maintained.
Another cooling solution is disclosed in publications EP2161620 and US 2007/0183156, where part of the LED tube is of metal which serves as a good heat conductor and transfers heat out of the LEDs. A problem with these cooling solutions using metal is that the metallic cooling part must be insulated sufficiently reliably from the LED circuits. Thus, sufficient insulation distances are required. If such LED tubes with a metallic cooling part are used such that they are fed by an electronic ballast, a high frequency, in particular, causes a further problem. That is to say, the conductor circuits of the LEDs generate stray capacitance in said metal cooling structure, which generates a capacitive leakage current. This leakage current may cause the risk of an electric shock which may even be life-threatening.